No documentation in Readme for HTML

[nabilfreeman]

I had to look at a closed issue for sending html. I know it's kind of a common sense thing, but it might be nice if there was a line in the Readme about sending HTML.

[akhoury]

this is just a client to the Mandrill API, the documentation of all requests are here:
https://mandrillapp.com/api/docs/

as for the html part see this
#11